Banana bunchy top virus (BBTV) poses a serious danger to banana crops worldwide. BBTV-encoded protein B4 is a determinant of pathogenicity. However， the relevant molecular mechanisms underlying its effects remain unknown. In this study， we found that a functional peptide could be liberated from protein B4， likely via proteolytic processing. Site-directed mutagenesis indicated that the functional processing of protein B4 is required for its pathogenic effects， including dwarfism and sterility， in plants. The released protein fragment targets host proteins， such as the large subunit of RuBisCO (RbcL) and elongation factor 2 (EF2)， involved in protein synthesis. Therefore， the peptide released from B4 (also a precursor) may act as a non-canonical modifier to influence host-pathogen interactions involving BBTV and plants.